(1) Field of the Invention
The present invention relates to a cross-connect method and a cross-connect apparatus suitable for use with a synchronous digital hierarchy (SDH) transmission system.
(2) Description of the Related Art
A ring network (hereinafter sometimes simply referred to as a “ring”) generally comprises a plurality of node devices (SDH transmission devices) connected together by means of high-speed transmission paths (for work and for protection).
Each of the node devices constituting the ring network [for example, an add-and-drop multiplexer (ADM)] is arranged so as to be able to receive a signal sent from another low-speed network of a lower hierarchical level by way of a low-speed transmission path and to add the signal to a high-speed transmission path (transmission path for work) so as to transmit the signal to another node device connected to the ring; to permit a signal—which is transmitted from another node device connected to the ring by way of the high-speed transmission path—to pass through to another node device; and to drop a signal to a low-speed network by way of a low-speed transmission path and through employment of drop control. An SDH frame of a higher-order group is transmitted over a high-speed transmission path provided in the ring network, and a signal received from a lower hierarchical level is inserted into a payload of the SDH frame.
In the field of the ring network, there has been conceived a uni-directional protection switched ring (UPSR) which, in the event of a failure in a transmission path over which a signal is transmitted from a certain node device, can switch a transmission path to another transmission path by means of automatic path switching (APS) control, as if the failure had not occurred.
In the UPSR-type ring network, identical signals are transmitted to a high-speed transmission path for work (hereinafter often referred to simply as a “path-for-work”) and a high-speed transmission path for protection (hereinafter often referred to simply as a “path-for-protection”), and a receiving apparatus monitors the quality of the signals (i.e., line quality) transmitted over the respective paths and, by means of a path switch, selectively outputs the signal having better quality.
In contrast with the UPSR-type ring network, there has been conceived a ring network of bi-directional line switched ring (BLSR) type. In the BLSR-type ring network, a signal is transmitted to the path-for-work at the time of normal operation. However, in the event of a failure, the signal which is being transmitted to the path-for-work is also transmitted to the path-for-protection.
In the BLSR-type ring network, a service selector of a receiving node device selects a signal—which is transmitted over the path for work—at the time of normal operation but selects a signal—which is transmitted over the path for protection—in the event of a failure, thus switching the transmission path between the path-for-work and the path-for-protection.
As mentioned above, the UPSR-type ring network and the BLSR-type ring network are now available. There has already been conceived a network system which is equipped with both the path switch and the service selector and which is compatible with both the ring UPSR-type network and the BLSR-type ring network.
For instance, a node device 10-1 shown in FIG. 11 is arranged so as to be compatible with both the UPSR-type ring network and the BLSR-type ring network. As shown in FIG. 11, the node device 10-1 comprises time slot assignment sections (TSA) 10-1a, 10-1b, and 10-1c; a path switch (PSW) 10-1d; a time slot interchange (TSI) section 10-1e; and service selectors (S.S) 10-1f and 10-1g. 
In the node device 10-1, the TSA sections 10-1a and 10-1b (soft switch) and the path switch (hard switch) change the path over which a signal is dropped from a line. After having been subjected to path switching operation, the signal is reduced in capacity to one-half as large as that of before cross-connect operation (i.e., soft switch).
The TSI section (cross-connect apparatus and soft switch) 10-1e and the service selectors 10-1f and 10-1g (hard switch) subject to service selector processing a signal added to the line or a signal passed therethrough, with the result that the signal is reduced in capacity to half that before the service selector processing.
The TSA sections 10-1a to 10-1c and the TSI section 10-1e perform operations only through line setting (soft switch) by means of software. As shown in FIG. 11, in a case where a hard switch, such as the path switch 10-1d or the service selectors 10-1f and 10-1g, is used, selection of the data on which a cross-connect operation has been performed (or that have been subjected to protection switching by use of a hard switch) is executed in connection with both a signal transmitted over the path for work and a signal transmitted over the path for protection.
In this case, the TSI section 10-1e performs a cross-connect operation on a received signal and transmits the thus-received signal to the service selectors 10-1f and 10-1g. In some cases, the signal on which a cross-connect operation has been performed is not selected by the service selectors 10-1f and 10-1g. Further, the PSW 10-1d is arranged to select a signal output from either the TSA 10-1a or the TSA 10-1b and outputs the thus-selected signal. A redundant configuration enables a cross-connect operation on a signal which is not output from the PSW 10-1d, the signal selector 10-1f, or the signal selector 10-1g, thereby raising a problem of excessive power consumption.